structural changes in polymers by means of cryogenic grinding › knowledge-base... · 196°c)...
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Grinding & Pulverizing
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Mixing & Blending
A P P L I C A T I O N N O T E Structural Changes in Polymers by Means of Cryogenic Grinding
With kind permission of Prof. Dr. U. Köster, Dept. of Chemical Engineering , University of Dortmund
One goal in materials research is to develop new synthetic materials with characteristics that
cannot be accomplished by the usual smelting process, as the polymers tend to separate during the
heating process.
Pure Polymer Trials to introduce permanent changes in polymers by means of Cryogenic Mechanical Milling
(CMM) with the Freezer/Mill® were started with two polymers; isotactic polypropylene (iPP) and
syndiotactic polystyrol (sPS). After melting and re-crystallization, both cryogenically ground (i.e.at–
196°C) polymers showed characteristics that differ from the original polymers. DSC (Differential
Scanning Calorimetry) tests showed changed crystallization temperatures, which indicates reduced
molecular weight or a change in molecular weight distribution. The longer the samples were
subjected to cryogenic grinding the bigger the change.
Further experiments showed that the crystalline structure is increasingly destroyed during
cryogenic grinding. Presumably CMM leads to chain fission and therefore to the formation of macro
radicals. This can lead to cross-linking and an increased amorphic content in the partially
crystallized polymers. Another explanation would be the formation of nano structures, small
fragments of the original polymer compounds.
Compounds As well as the pure polymer results the behavior of cryogenically ground polymer compounds is, of
course, very interesting. When mixing the two polymers iPP and sPS through melting (extrusion)
separation occurs. Such polymer blends are dispersions of one part within the other. Some CMM
sample tests suggest that re-combination of constitutive different macro radicals partly results in
co-polymers. These co-polymers could act as phasing facilitators between the incompatible
blending components of iPP and sPS. Evidence of this is the altered morphology of CMM blends.
Here the DSC experiments also showed changed crystallization behavior. More graphic are the
morphology light microscopy pictures. While, after melting and re-crystallization, the extruded
samples show morphology characteristics for roughly separated polymer blends, in a CMM sample
the iPP is finely distributed within the sPS. (See Fig.)
Conclusion CMM using the Freezer/Mill causes changes to structure, thermal behaviour and morphology of
homopolymers and polymer compounds. While morphological differences through melting and re-
crystallization are partly reversible, structural changes remain. Aside of chain fission and the
resulting macro radicals, cross-linking and reduced crystallization level has been observed. Polymer
blends produced by CMM show a considerably improved dispersion of both polymers within each
: : APPLICATION NOTE SP008:
Structural Changes in Polymers
: : APPARATUS:Freezer/Mill®
: : APPLICATION:Polymer Blending
SPEX SamplePrep
65 Liberty St
Metuchen, NJ 08840 USA
Tel: 732-623-0465
Fax: 732-906-2492
E-mail: [email protected]
www.spexsampleprep.com
European Headquarters
SPEX Europe
2 Dalston Gardens
Stanmore, HA7 1BQ, UK
Tel: +44 (0) 208 204 6656
Fax: +44 (0) 208 204 6654
E-mail: spexeurope.com
Web: www.spexeurope.com
other. Exactly this changed degree of dispersion gives reason to hope for new materials with new
and perhaps even predictable characteristics.